Much construction in modern economies lie in building, installing, and maintaining surface and subsurface structures such as roads, water distribution, drainage systems, pipelines, barriers, fences, electrical transmission infrastructure, telecommunication infrastructure, and the like. In cold climates, frost persists in the ground for much of the year rendering summertime equipment for penetrating the surface (e.g., digging, trenching, ploughing, filling, sealing, and the like) ineffective without first thawing the ground. This circumstance is particularly pronounced in the urban environment where frost is typically much deeper and precise dimensionality of subsurface penetration much more critical. Many construction techniques are available for thawing the top 10 centimetres of ground fairly efficiently. However few services are buried at such depths and many services are situated deep within the frost zone. Consequently, controlling the dimension of the thaw has become increasingly important as too little thawing at the required dimension and depth leads to difficulty moving the earth as desired while too much thawing may cause problems such as wasted energy or sloughing of the adjacent terrain.
Concurrently with the proliferation of subsurface structures that has occurred in the past 60 years, there has been an evolution of regulations and standards in the construction industry arising from improved understanding in the engineering, occupational safety, environmental, urban-planning, fire-safety, and allied fields. Meeting these regulations is often challenging and expensive. In order to achieve operating profitably within these evolving limitations contractors have had to investigate new ways to achieve their ends
Heat transfer for thawing is typically accomplished by a combination of conduction, convection, and radiation. Conventionally, ground heating or thawing is typically undertaken by 1) piping heated fluids (e.g., glycerol) through hoses having a serpentine configuration disposed under thermal blankets or soil, 2) heating enclosed air over a construction site, 3) placing a portable heating enclosure over the target ground or 4) burning materials (usually a coal-straw mixture) over the ground to be thawed.
Serpentining piping filled with heated fluids under thermal blankets (e.g. Grochoski, U.S. Application No. 2003/0124315) or within mats (e.g., Albert, U.S. Application No. 2010/0119306) are designed mainly for surface heating and curing of concrete. For trenching, pipes or tubes are sometimes buried to gain the transmission and insulating effects. For curing of concrete, the blankets absorb significant amount of the heat output providing a relatively uniform lateral heat distribution for the air under the blankets. When used for deep thawing, the downward radiation and conduction is a relatively small part of the energy output; thus, the technique can be slow and may result in uneven thawing at target depths. Moreover, this technique can lead to significant loss of energy over the length of the hoses or pipes, especially when the heat source is far from the thaw zone. Also, while thaw zones are characteristically targeted as right angular plans, hoses are typically of different size than the target zone and must be laid out in hairpins to approximate the layouts of these planned construction zones. Uneven distances between these conduits may also result in uneven heating throughout the target thaw zone. At any given construction site, one or more of these limitations may result in difficulty in planning or meeting schedules.
Similarly, when one heats an indoor air environment inside of shrouding or a canopy, the working environment may comfortable and enclosed surfaces compliant to best practices for curing, sealing and the like but ground thawing is superficial and normally not dimensionally compliant to the enclosure at depth. The shape of the subsurface thaw will also be deepest in the middle while achieving very little thawing at the edges of the thaw zone. Investigators have tried to use general-purpose construction heaters for blowing radiant heat to warm the air (e.g., Schmidt, U.S. Pat. No. 4,682,578) or canopies with suspended heating devices (e.g., Nielson et al., U.S. Application No. 2005/0103776) achieved some thawing but had difficulty with deep thawing. These methods are sometimes even impotent for frost deeper than 20 cm. This result may be magnified in harsh conditions as the susceptible to the elements of weather wherein colder or faster moving air absorbs the energy the contractor wants focused on the target ground. Again, any of these complications may result in a contractor having difficulty planning or meeting schedules.
As an alternative, certain devices are sold that provide a propane burner with a case or outer housing. U.S. Pat. No. 5,033,452 (issued to Carriere) theorizes that liquid water on the ground surface is a major impediment to ground thawing and that removal is an improvement in efficiency. Carriere discloses a thawing device having a thermally insulated housing and a single undivided fire tube mounted within the housing. The fire tube has a first end connected to one port in the housing and a second end connected to another port in the housing. A burner is mounted outside the housing in the first end of the fire tube, that tube running along the ground surface, and a flue for exhausting the combustion gases is connected adjacent the second end. Heat transmitted from the fire tube directly into the ground and interior of the housing serving to evaporate water. The housing includes a steam vent to provide egress for the moisture. Carriere does not concern himself with the evenness of the thaw within the device or how his devices may be used in collaboration to achieve an intended result.
Another ground thawing-device, called “Frost Hog,” is manufactured by Leric Holdings, Ltd., of Lloydminster, Alberta, Canada. The device includes a heavy trailer-mounted housing and a fire tube extending through the housing from one port to another port. A burner is positioned in the first port and a vertical flue for exhausting the combustion gases is positioned adjacent the second port. Because of its size and trailer mount, the unit is difficult to place between structures (for example between a garage and a fence) and cannot be used in contiguous arrays that would thaw ground for contiguous underground structures such as gas or electrical service.
Yet another ground-thawing device, called the “Thaw Dawg”, is manufactured by Ground specialties Incorporated of Minneapolis, Minn., U.S.A. The device comes with a 36″×48″ case with an open bottom and provides a burner attached to one of the 48″ sides of the case. The external burner limits its use near building structures and trees and results in the production of waste heat. Even though the burner is relatively close to all parts of the enclosure, in our hands, the heat and thawing is most intense directly below the burner and thawing underground occurs in an inverted, non-circular, conical fashion. Accordingly, this device does not predictably allow trenching of the entire dimension of the case footprint in a period less than 48 top 72 hours. Moreover, if placed end to end to enable the digging of a 48″ wide trench, for example, the external burner box and inverted conical thawing at depth would result in intermittent segments where there is difficult digging in frozen ground.
Consequently, for many years trenching contractors almost universally burned mixtures of coal and straw laid out along a trench-line to thaw terrain for digging on subsequent days. This technique was not without drawbacks. When temperature dropped rapidly overnight, the inability to capture heat and direct the heat downward often resulted in incomplete thawing within the production schedule. This technique also suffered from intermittent loss of ignition by vandalism, rain, snow, melt water or discontinuities in fuel as well as pollution through emission of smoke, cinders, and odor. Accordingly, contractors needed to employ personnel for monitoring the burn over extended periods of time. Even with monitoring, the combination wind and cinders, left an ever-present fire hazard. Accordingly, this technique was particularly unsafe for use near construction equipment, buildings, as well as in dry fields, or wooded areas. If there were delays between burning and trenching, the local microenvironment was uncontrolled resulting in the potential for refreezing. For these fire and environmental reasons, using unattended burning materials for ground preparation is a practice now banned in many jurisdictions. Nonetheless, this method forms the “gold standard” for efficacy against which all other methods are measured.
Accordingly, all conventional deep-thawing practices suffer the common problem of scheduling reliably. Under well-controlled conditions, the method of burning a straw-coal mixture along a trench-line typically achieved a centre-line deep thaw of approximately 3 feet or 1 meter in 72 hours where there has been a successful burn. Depending on the outside air temperature, the use of construction heaters in a tarped-in or canopied area may achieve one half of that depth in a similar period along the centre-line of the structure. Using hydronic heaters with insulating blankets would typically achieve a thawing result somewhere between these two methods. Efficacy of various portable inventions is highly variable depending upon the task assigned. None of the above-mentioned methods reliably leave a dimensionally uniform thaw zone at a predictable time. Accordingly, the more spatially complex the target thaw zone becomes, the more refractive scheduling becomes for any given subsurface construction activity.
It is, therefore, desirable to provide an apparatus and method for thawing frozen ground that overcomes the shortcomings of the prior art.